Jean-Claude Thelen

The Havemann‐Taylor Fast Radiative Transfer Code: Exact fast radiative transfer for scattering atmospheres using Principal Components (PCs)

The Havemann‐Taylor Fast Radiative Transfer Code (HT‐FRTC) has been developed for the simulation of highly spectrally resolved measurements from satellite based (i.e. Infrared Atmospheric Sounding Interferometer (IASI), Atmospheric Infrared Sounder (AIRS)) and airborne (i.e. Atmospheric Research Interferometer Evaluation System (ARIES)) instruments. The use of principle components enables the calculation of a complete spectrum in less than a second. The principal compoents are derived from a diverse training set of atmospheres and surfaces and contain their spectral characteristics in a highly compressed form. For any given atmosphere/surface, the HT‐FRTC calculates the weightings (also called scores) of a few hundred principal components based on selected monochromatic radiative transfer calculations, which is far cheaper than thousands of channel radiance calculations. By intercomparison with line‐by‐line and other fast models the HT‐FRTC has been shown to be accurate. The HT‐FRTC has been successfully ...

Surface retrievals from Hyperion EO1 using a new, fast, 1D-Var based retrieval code

We have developed a new algorithm for the simultaneous retrieval of the atmospheric profiles (temperature, humidity, ozone and aerosol) and the surface reflectance from hyperspectral radiance measurements obtained from air/space-borne, hyperspectral imagers such as Hyperion EO-1. The new scheme, proposed here, consists of a fast radiative transfer code, based on empirical orthogonal functions (EOFs), in conjunction with a 1D-Var retrieval scheme. The inclusion of an ’exact’ scattering code based on spherical harmonics, allows for an accurate treatment of Rayleigh scattering and scattering by aerosols, water droplets and ice-crystals, thus making it possible to also retrieve cloud and aerosol optical properties, although here we will concentrate on non-cloudy scenes. We successfully tested this new approach using hyperspectral images taken by Hyperion EO-1, an experimental pushbroom imaging spectrometer operated by NASA.

The Havemann-Taylor Fast Radiative Transfer Code: A line-by-line sensor independent Radiative Transfer Code

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is based on Principal Components (PCs) and allows fast and exact radiance and/or transmittance calculations. It is ideally suited for the simulation of hyperspectral sensors with hundreds or thousands of channels. The HT-FRTC can simulate a full instrument spectrum for any atmosphere and surface within a few milliseconds. It works for satellite-based, airborne and ground-based sensors. The code has been applied in any part of the spectrum from the short-wave to the long-wave (i.e. infrared plus microwave). It includes the solar and the lunar source and can account for the spherical Earth. The HT-FRTC has been incorporated into a one-dimensional variational (1D-Var) retrieval system that also works solely in PC space. This keeps the dimensions of the matrices involved small. The solution of the full non-linear problem is achieved by an iterative Levenberg-Marquardt minimization procedure. The retrieval state vector includes the vertical profiles of atmospheric temperature, water vapour and ozone, and possibly other trace gases as well as the surface temperature and surface emissivity / reflectivity (the latter being represented by a set of PCs). For a scattering atmosphere, cloud parameters and aerosol parameters have been added to the state vector. The cloud part of the state vector for cirrus cloud includes cloud-top pressure, ice water content, cloud fraction and cloud geometrical thickness. For water cloud there is also an effective droplet size.

HT-FRTC: a fast radiative transfer code using kernel regression

The HT-FRTC is a principal component based fast radiative transfer code that can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra. The principal components cover the spectrum at a very high spectral resolution, which allows very fast line-by-line, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from line-by-line simulations for a diverse set of atmosphere and surface conditions. The derived principal components are sensor independent, i.e. no extra training is required to include additional sensors. During the training phase we also derive the predictors which are required by the fast radiative transfer code to determine the principal component scores from the monochromatic radiances (or fluxes, transmittances). These predictors are calculated for each training profile at a small number of frequencies, which are selected by a k-means cluster algorithm during the training phase. Until recently the predictors were calculated using a linear regression. However, during a recent rewrite of the code the linear regression was replaced by a Gaussian Process (GP) regression which resulted in a significant increase in accuracy when compared to the linear regression. The HT-FRTC has been trained with a large variety of gases, surface properties and scatterers. Rayleigh scattering as well as scattering by frozen/liquid clouds, hydrometeors and aerosols have all been included. The scattering phase function can be fully accounted for by an integrated line-by-line version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling).

HT-FRTC: A fast radiative transfer code using kernel regression

The HT-FRTC is a principal component based fast radiative transfer code that can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra. The principal components cover the spectrum at a very high spectral resolution, which allows very fast line-by-line, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from line-by-line simulations for a diverse set of atmosphere and surface conditions. The derived principal components are sensor independent, i.e. no extra training is required to include additional sensors. During the training phase we also derive the predictors which are required by the fast radiative transfer code to determine the principal component scores from the monochromatic radiances (or fluxes, transmittances). These predictors are calculated for each training profile at a small number of frequencies, which are selected by a k-means cluster algorithm during the training phase. Until recently the predictors were calculated using a linear regression. However, during a recent rewrite of the code the linear regression was replaced by a kernel regression which resulted in a significant increase in accuracy when compared to the linear regression. The HT-FRTC has been trained with a large variety of gases, surface properties and scatteres. Rayleigh scattering as well as scattering by frozen/liquid clouds, hydrometeors and aerosols have all been included. The scattering phase function can be fully accounted for by an integrated line-by-line version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling). Typically the simulation of a whole clear-sky radiance spectrum (3600000 monochromatic frequencies) takes less than one millisecond.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) and its application within Tactical Decision Aids (TDAs)

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is a core component of the Met Office NEON Tactical Decision Aid (TDA). Within NEON, the HT-FRTC has for a number of years been used to predict the infrared apparent thermal contrasts between different surface types as observed by an airborne sensor. To achieve this, the HT-FRTC is supplied with the inherent temperatures and spectral properties of these surfaces (i.e. ground target(s) and backgrounds). A key strength of the HT-FRTC is its ability to take into account the detailed properties of the atmosphere, which in the context of NEON tend to be provided by a Numerical Weather Prediction (NWP) forecast model. While water vapour and ozone are generally the most important gases, additional trace gases are now being incorporated into the HT-FRTC. The HT-FRTC also includes an exact treatment of atmospheric scattering based on spherical harmonics. This allows for the treatment of several different aerosol species and of liquid and ice clouds. Recent developments can even account for rain and falling snow. The HT-FRTC works in Principal Component (PC) space and is trained on a wide variety of atmospheric and surface conditions, which significantly reduces the computational requirements regarding memory and processing time. One clear-sky simulation takes approximately one millisecond at the time of writing. Recent developments allow the training of HT-FRTC to be both completely generalised and sensor independent. This is significant as the user of the code can add new sensors and new surfaces/targets by supplying extra files which contain their (possibly classified) spectral properties. The HT-FRTC has been extended to cover the spectral range of Photopic and NVG sensors. One aim here is to give guidance on the expected, directionally resolved sky brightness, especially at night, again taking the actual or forecast atmospheric conditions into account. Recent developments include light level predictions during the period of twilight.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) and its applications

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is a component of the Met Office NEON Tactical Decision Aid (TDA). Within NEON, the HT-FRTC has for a number of years been used to predict the IR apparent thermal contrasts between different surface types as observed by an airborne sensor. To achieve this, the HT-FRTC is supplied with the inherent temperatures and spectral properties of these surfaces (i.e. ground target(s) and background). A key strength of the HT-FRTC is its ability to take into account the detailed properties of the atmosphere, which in the context of NEON tend to be provided by a Numerical Weather Prediction (NWP) forecast model. While water vapour and ozone are generally the most important gases, additional trace gases are now being incorporated into the HT-FRTC. The HT-FRTC also includes an exact treatment of atmospheric scattering based on spherical harmonics. This allows the treatment of several different aerosol species and of liquid and ice clouds. Recent developments can even account for rain and falling snow. The HT-FRTC works in Principal Component (PC) space and is trained on a wide variety of atmospheric and surface conditions, which significantly reduces the computational requirements regarding memory and time. One clear-sky simulation takes approximately one millisecond. Recent developments allow the training to be completely general and sensor independent. This is significant as the user of the code can add new sensors and new surfaces/targets by simply supplying extra files which contain their (possibly classified) spectral properties. The HT-FRTC has been extended to cover the spectral range of Photopic and NVG sensors. One aim here is to give guidance on the expected, directionally resolved sky brightness, especially at night, again taking the actual or forecast atmospheric conditions into account. Recent developments include light level predictions during the period of twilight.